Generally, the present invention relates to optical gain elements based on semiconductor laser diodes, and particularly to laser diode gain elements having curved waveguides.
Optical semiconductor devices produce large reflections due to the high refractive index of the semiconductor material. Anti-reflection coatings are typically employed to reduce the amplitude of the reflections, preferably to a level of less than 0.1%. However, an anti-reflection coating for a semiconductor device is a relatively complex coating due to the semiconductor's high refractive index. Consequently, the reflectivity of an anti-reflection coating is generally low only over a small range of wavelengths, with the reflectivity increasing for wavelengths outside this small range.
Problems may arise, for example, where a semiconductor gain element is employed in a tunable, external cavity laser, where anti-reflection coatings are typically required on intracavity surfaces, including any intracavity face of the gain element. The tuning range over which the laser can operate is limited because the anti-reflection coatings on the gain element are effective only over a narrow range of wavelengths. Operation of the laser outside the wavelength range where the gain element's anti-reflection coating is most effective may result in spurious reflections compromising the quality of the output signal from the laser. Furthermore, the operating power of the semiconductor gain element is limited, since the higher reflectivity for wavelengths outside the range of the anti-reflection coating may result in uncontrolled, untunable oscillation at those wavelengths, or may result in some other degradation of the quality of the output from the laser.
Therefore, there is a need to increase the tuning range over which external cavity semiconductor lasers may be tuned, and also a need to permit external cavity, tunable lasers to operate at high power without compromising the quality of the laser output.